One would think that people would have been taught in their Language Arts classes (for whatever language) that taking someone else's work and slapping their name on it was stealing - they wouldn't have gotten away with it in school, so why do they think they can get away with it in the rest of their lives?
It also means they haven't read any of the guidelines ff.net has on what is and is not allowed, nor any other archive guidelines.
Luckily, such blatant theft seems relatively rare in fanfiction - in part due to self-policing as evidenced here.
